1. Field of the Invention
The present invention generally relates to ground resistance monitoring devices, and more particularly to a ground resistance monitoring device capable of monitoring ground resistances of multiple work stations simultaneously in real time.
2. The Prior Arts
Grounding is often considered an extremely important issue in preventing electrostatic hazard. However, despite the technology advances, achieving a reliable and effective grounding still seems a difficult job. Accidents due to improper grounding are still quite often, even for high-tech manufacturers. Electrostatics, if not treated seriously and cautiously, could cause severe property loss and human lives too.
In a conventional work place, there are usually multiple assembly lines each containing a number of assembly stations or machines. To prevent the electrostatics carried by the operators working in these assembly lines from damaging the products in production, the operators are usually required to wear electrostatic-proof clothes, shoes, and/or wrist strap, and the floor and the table surface are usually covered with floor mats or table mats to prevent the accumulation of electrostatics. As shown in FIG. 1a, the floor mat 10, the table mat 20 and the wrist strap (not shown) are usually connected to a common point ground 30 in an assembly station. The common point ground 30 is usually a metallic plate or wire set up at a fixed location in the assembly station.
As shown in FIG. 1b, the common point grounds 30 of the assembly stations in an assembly line are series-connected by a conductive wire 40, and the conductive wires 40 of separate assembly lines are jointed together or separately connected to either an equipment ground or an earth ground (independent of the equipment ground).
The equipment ground is usually the ground provided by the ground wire of the AC (alternate current) mains. When a building is constructed, one or more ground bars are hammered into the ground to form a ground net 70. The ground wire of the AC mains is then connected to the ground net 70. The ground of the AC mains is then branched to the power outlets through the panel box 50 at each floor and a major panel box 60 of the building. The earth ground is usually connected to an additional and independent ground bar or ground net. In either way, electrostatics carried by or accumulated on the operators are conducted to earth through the wrist straps, the table mats, the floor mats, to the common point grounds in assembly stations, and then to the equipment ground of the building or the independent earth ground, so as to prevent potential electrostatic hazard.
The foregoing grounding structure is also commonly found in production lines with a number of working machines, such as the surface mounting machines, and these working machines are usually connected to the equipment ground through their power cables respectively. Besides assembly lines, the service islands of a gas station have a similar environment. Each service island has a number of gasoline pumps each of which has several hoses to provide different kinds of gasoline. The nozzles of the hoses are connected to a conductive wire of the gasoline pump, and the conductive wires of the gasoline pumps of a service island is series-connected to a ground bar or a ground net of the gas station.
For assembly lines, production lines, service islands, or any other similar working environments, they all contain a number of work stations (hereinafter, the term is used to refer to assembly stations, machines, gasoline pumps, or any other similar spots requiring appropriate grounding) which should be properly grounded to prevent electrostatic hazard. However, it is quite often that these ground bars or ground nets are rust-eaten to provide proper grounding, or the panel boxes are not connected to the ground bar or ground net for some reason, or the conductive wires are broken. All these would cause one or more work stations to have inadequate grounding and therefore the electrostatics start to accumulate on the operators which will damage the product under production or even the expensive machines.
Conventionally, a common approach to the problem is to conduct periodic manual examination of the grounding of each work station. One manual examination method is the so-called three-point-grounding method by a specialized ground resistance measurement equipment. The three-point-grounding method is to connect two test leads of the equipment to two existing ground bars and then connect a third test lead of the equipment to the ground bar or ground net under test. As can be imagined, this method requires quite some effort and time, and only applicable to a limited number of environments. For ground points indoors or at higher floor locations, this method is not adequate. Moreover, this kind of periodic examination still cannot detect and resolve grounding problems happening during the window between two examinations.
There are quite a few other approaches proposed in the related arts. For example, a wrist strap disclosed in U.S. Pat. No. 5,623,255 would siren when it is disconnected from the common point ground. However, whether the common point ground indeed provides adequate grounding is still unknown. A monitoring device disclosed in R.O.C. (Taiwan) Patent No. 448,414 can automatically monitor and issue alarm when detecting abnormal grounding for a single ground point. To monitor multiple grounding points, multiple devices have to be prepared which is too costly and, on the other hand, lacks the convenience of centralized management.
Additionally, the existing techniques usually require a ground that is known to be appropriate as a reference. To locate and make sure the reference ground is indeed appropriate, the foregoing methods such as the three-point-grounding method has to be used, which adds a lot of trouble in setting up the monitoring devices. What is even more bothersome is that whether the reference ground remains effective may be an issue by itself and therefore may require periodic examination as well.